linux-kernel - Re: [PATCH 5/5] drm/sun4i: Run the mixer clock at 297 MHz on V3s

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Message-ID: <20250702-psychedelic-stalwart-jerboa-a626eb@houat>
Date: Wed, 2 Jul 2025 13:36:53 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Paul Kocialkowski <paulk@...-base.io>
Cc: linux-media@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	linux-clk@...r.kernel.org, dri-devel@...ts.freedesktop.org, linux-gpio@...r.kernel.org, 
	Yong Deng <yong.deng@...ewell.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Linus Walleij <linus.walleij@...aro.org>, Icenowy Zheng <icenowy@...c.xyz>, 
	Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH 5/5] drm/sun4i: Run the mixer clock at 297 MHz on V3s

On Tue, Jul 01, 2025 at 10:11:24PM +0200, Paul Kocialkowski wrote:
> The DE mixer clock is currently set to run at 150 MHz, while the
> Allwinner BSP configures it at 300 MHz and other platforms typically
> run at 297 MHz.
> 
> 150 MHz appears to be enough given the restricted graphics capabilities
> of the SoC (with a work area of only 1024x1024). However it typically
> causes the DE clock to be parented to the periph0 pll instead of the
> video PLL.
> 
> While this should generally not be a concern, it appears (based on
> experimentation) that both the DE and TCON clocks need to be parented
> to the same PLL for these units to work. While we cannot represent this
> constraint in the clock driver, it appears that the TCON clock will
> often get parented to the video pll (typically running at 297 MHz for
> the CSI units needs), for instance when driving displays with a 33 MHz
> pixel clock (33 being a natural divider of 297).
> 
> Running the DE clock at 297 MHz will typically result in parenting to
> the video pll instead of the periph0 pll, thus making the display
> output functional.
> 
> This is all a bit fragile but it solves the issue with displays running
> at 33 Mhz and brings V3s to use the same frequency as other platforms,
> making support more unified.

It's beyond fragile, and doesn't have anything to do with the DRM driver.

You should set up the clock tree properly in the clock driver, and then
add NO_REPARENT to the DE clock to make sure it stays that way.

And then, you can change the clock rate if you want to, but at least you
don't set a rate and hope that the side effects work your way, and won't
happen again.

Maxime

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